He made the remarks during a visit to the Armenian capital, Yerevan, for commemorations of the massacre.

Armenia and many historians say that up to 1.5 million Armenian Christians were killed by Ottoman forces in 1915.

Turkey has always disputed that figure and rejects the use of the term "genocide". It says the deaths were part of a civil conflict triggered by the war.

The row over how to characterise the killings has continued to sour relations between Armenia and Turkey, as well as drawing in other countries, such as Germany, whose parliament recently declared the killings to be genocide.

In an address to Armenia's president Serzh Sargsyan and the diplomatic corps, the Pope appeared to have added the word "genocide" to his prepared text.

"This tragedy, this genocide, has unfortunately marked the start of a sad series of great catastrophes of the last century," he said.

There was no immediate reaction from Turkey, which last year recalled its envoy to the Vatican after the Pope referred to "genocide".

In 2014, for the first time, Turkish prime minister Recep Tayyip Erdogan offered condolences to the grandchildren of all the Armenians who lost their lives.

But he also said that it was inadmissible for Armenia to turn the issue "into a matter of political conflict".

Armenia says that up to 1.5 million people died in 1915-16 as the Ottoman Empire was disintegrating. Turkey has said that the number of deaths was much smaller.

Many of the victims were civilians deported en masse to barren desert regions, where they died of starvation and thirst. Thousands also died in massacres.

Most non-Turkish scholars of the events regard them as genocide.

Among the other states which formally recognise them as genocide are Argentina, Belgium, Canada, France, Italy, Russia and Uruguay.